1. Field of the Invention
The present invention relates to a wafer grinding method for grinding a wafer having crystal orientation.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of rectangular regions where devices such as ICs and LSIs are respectively formed. The semiconductor wafer having many devices as mentioned above is divided along these streets to thereby obtain individual semiconductor chips. Also in the case of a wafer composed of a substrate of lithium tantalate, for example, and a plurality of piezoelectric elements provided in the substrate, the wafer is cut along predetermined streets to obtain individual chips, which are widely used in electrical equipment.
To reduce the size and weight of each chip, the back side of the wafer is usually ground to reduce the thickness of the wafer to a predetermined thickness prior to dividing the wafer along the streets to obtain the individual chips. Further, a so-called early dicing is generally performed as a wafer dividing method such that the wafer is not fully cut into the individual chips by a cutting apparatus, but a groove having a predetermined depth corresponding to the finished thickness of each chip is formed along each street on the front side of the wafer, and the back side of the wafer is next ground until the bottom of each groove is exposed to the back side of the wafer.
It is known that a grinding apparatus for grinding the back side of the wafer includes a chuck table for holding the wafer as a workpiece and grinding means having an annular grinding wheel for grinding the upper surface (back side) of the wafer held on the chuck table, wherein the back side of the wafer is ground by rotating the chuck table, rotating the grinding wheel, and feeding the grinding wheel so that the lower end surface or grinding surface of the grinding wheel is passed through the center of the wafer held on the chuck table (see Japanese Patent Laid-open No. 2000-354962, for example).
According to the grinding method described in Japanese Patent Laid-open No. 2000-354962 mentioned above, the wafer can be efficiently ground to obtain a predetermined thickness. However, as the result of measurement of the die strength of each chip obtained by dividing the wafer after such back grinding, it has been found that some chips having a remarkably low die strength are quantitatively present. More specifically, in the case that the wafer is ground by the grinding method described in Japanese Patent Laid-open No. 2000-354962, a saw mark is formed on the ground surface of the wafer so as to extend radially from the center of the wafer to the outer circumference thereof. In relation to the crystal orientation of the wafer, some chips are quantitatively present in a region where the saw mark extends in an easily breakable direction, so that some chips having a remarkably low die strength are quantitatively generated. It is known that the region where the chips having a low die strength are quantitatively generated is a region where a mark for indicating the crystal orientation of the wafer is in a predetermined relation to the saw mark (in the case of a silicon wafer, the saw mark extends at 45° with respect to the mark indicating the crystal orientation).
To solve the above problem, there has been proposed a wafer grinding method including a first grinding step and a second grinding step. The first grinding step is performed by rotating a chuck table holding a wafer, rotating a grinding wheel, and feeding the grinding wheel so that the lower end surface of the grinding wheel is passed through the center of the wafer, thereby grinding the upper surface of the wafer held on the chuck table. After performing the first grinding step, the second grinding step is performed by moving the chuck table holding the wafer to a position spaced sideways from the grinding wheel to point the mark indicating the crystal orientation in a predetermined direction, next feeding the grinding wheel by a predetermined amount to a grinding position, and next relatively moving in parallel the chuck table holding the wafer and the grinding wheel being rotated at the grinding position, thereby grinding the upper surface of the wafer from the outer circumference of the wafer in a predetermined direction (see Japanese Patent Laid-open No. 2005-28550, for example).
In the wafer grinding method disclosed in Japanese Patent Laid-open No. 2005-28550 mentioned above, the outer circumferential surface of the wafer comes into impactive contact with the grinding wheel in the second grinding step, causing a possibility of chipping of the wafer.